Complete Heart Block: Emergency Recognition and Management

A 76-year-old woman taking metoprolol and diltiazem for atrial fibrillation is brought in after two dizzy spells at home. Her pulse is 32, blood pressure 82/50, and she has had diarrhoea for three days. The monitor shows regular, broad complexes with P waves marching through them independently. Before reaching for a pacing wire, the physician needs answers to three questions: is the block nodal or infranodal, is there a reversible cause, and how much time is left?

Key points

• Complete heart block means AV dissociation: atrial and ventricular activity are independent, with the atrial rate faster than the ventricular rate.
• A narrow escape rhythm suggests block at the AV node; a broad escape usually means infranodal block, which is slower, less reliable and less responsive to atropine.
• Always look for reversible causes: inferior MI, hyperkalaemia, AV nodal blocking drugs and Lyme carditis.
• Atropine, transcutaneous pacing and isoprenaline or adrenaline infusions are bridges; temporary transvenous pacing is the reliable bridge.
• Complete heart block without a reversible cause needs referral for permanent pacing.

Recognition

In third-degree AV block no atrial impulses are conducted to the ventricles. On the ECG, P waves appear at a regular rate, QRS complexes appear at a slower regular rate, and the PR interval varies randomly because the two rhythms are unrelated. Callipers set to the P–P interval will show P waves hidden in QRS complexes and T waves. Two look-alikes are worth excluding: 2:1 AV block, in which every second P wave conducts with a constant PR interval, and isorhythmic AV dissociation, in which the ventricular rate equals or exceeds the atrial rate.

Assess urgency

Resuscitation algorithms identify adverse features: shock, syncope, myocardial ischaemia and heart failure. Separately, the risk of asystole is higher with a recent episode of asystole, Mobitz II block, complete heart block with a broad QRS, or ventricular pauses longer than three seconds. Patients with any of these need pacing capability at the bedside, even if they look stable at the moment.

Reversible causes to look for first

• Inferior MI: block is usually at the AV node, with a narrow escape, and is often transient. Reperfusion is the treatment.
• Anterior MI: block reflects extensive septal necrosis, the escape is broad, and the prognosis is worse. Pacing is usually needed.
• Hyperkalaemia: check potassium urgently in every case. Combined AV nodal blocking drugs, kidney injury and hyperkalaemia can reinforce one another and are sometimes described together as BRASH syndrome.
• Drugs: beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem) and digoxin, alone or in combination; also consider amiodarone.
• Lyme carditis: consider it in patients with a relevant exposure history; conduction usually recovers with antibiotics.
• Other causes: infective endocarditis with aortic root abscess (new PR prolongation or block is a warning sign), recent cardiac surgery or TAVI, hypothyroidism, infiltrative disease such as sarcoidosis.

Complete heart block is one of the ten conditions in our pillar guide to the cardiac emergencies every physician must know.

Atropine and its limits

For an unstable bradycardia, atropine remains the first drug. The European Resuscitation Council recommends 500 micrograms IV, repeated every 3–5 minutes to a maximum of 3 mg; the AHA algorithm uses 1 mg doses up to the same maximum. Atropine acts on the sinus and AV nodes by blocking vagal tone. It therefore helps in nodal block but is generally ineffective in infranodal block with a broad escape, and occasionally worsens conduction by speeding the atrial rate. It should not be relied upon in a denervated transplanted heart. If there is no prompt response, move on rather than giving repeated doses.

Transcutaneous pacing

Apply the pads, ideally anteroposteriorly, set demand mode at a rate of 60–70/min, and increase the current until electrical capture is seen. Then confirm mechanical capture by feeling the femoral pulse, as skeletal muscle twitching can mimic a carotid pulse. Transcutaneous pacing is painful; give analgesia and sedation. It is a short bridge, and capture can be lost without warning, so arrange transvenous pacing in parallel.

Chronotropic infusions

• Isoprenaline, starting at about 5 micrograms/min and titrated to response, can increase the escape rate. Use it cautiously in active ischaemia.
• Adrenaline infusion, 2–10 micrograms/min, is an alternative, particularly when hypotension also needs support.
• Dopamine is included as an alternative in AHA algorithms.

Treat drug-induced block specifically where possible: digoxin-specific antibody fragments for digoxin toxicity, and specialist toxicology advice on agents such as glucagon, IV calcium and high-dose insulin therapy for beta-blocker or calcium channel blocker poisoning.

Temporary transvenous pacing

Temporary transvenous pacing is indicated when bradycardia remains haemodynamically significant despite these measures, when the escape rhythm is unreliable, or when a reversible cause will take time to resolve. The right internal jugular and femoral veins are common routes; many operators avoid the left subclavian vein to preserve it for a permanent device. Use fluoroscopy or echo guidance where possible, check the pacing threshold and lead stability, and watch for complications such as perforation, infection and loss of capture. Do not insert a wire reflexively when correcting potassium or stopping a drug will restore conduction within hours.

Permanent pacemaker referral

ESC 2021 pacing guidance supports permanent pacing for third-degree AV block not attributable to a reversible cause, regardless of symptoms. After acute MI, AV block often resolves, and guidelines advise a period of observation before deciding. In Lyme carditis, permanent pacing is generally avoided during the acute phase. Block after TAVI or cardiac surgery should be discussed with the implanting team. Patients who meet criteria should be referred promptly rather than discharged with “watch and wait” advice.

The traps

• Giving repeated atropine to a broad-complex escape rhythm.
• Missing hyperkalaemia or drug toxicity and placing an avoidable pacing wire.
• Assuming electrical capture on the monitor means the heart is being paced.
• Discharging a patient with unexplained complete heart block because the symptoms have settled.

For a structured, case-based approach to bradyarrhythmias and other emergencies, explore the 12-week Practical Cardiology Certificate for Physicians; the founding cohort begins on 23 November 2026.

This article is educational content intended for registered medical practitioners. It does not replace clinical judgement. Always follow your local protocols and current national and international guidelines when managing patients.